In-line skate wheels require elastomers having excellent resilience (rebound), which prevents the wheels from overheating or even melting during use. In addition, highly resilient wheels roll more easily, so they require less effort from the skater.
High-performance polyurethane cast elastomers (typically made from MDI, polytetramethylene ether glycol (PTMEG), and 1,4-butanediol) are currently the material of choice for in-line skate wheels. The best (and usually the most expensive) elastomers use PTMEG and have resiliences of 70-80%. Less expensive elastomers can be made with polyester polyols or polyoxypropylene diols (PPGs) instead of PTMEG, but these have significantly lower resilience. For example, a typical elastomer based on conventional PPGs has a resilience of only 50-55%. The industry would benefit from inexpensive elastomers having high resilience.
Opaque skate wheels are spinning out of favor with skaters and skateboarders. Like baggy jeans and heavy metal, clear wheels are "in." Skaters just like the looks of transparent wheels. Unfortunately, to make clear wheels, formulators usually have to make composition changes that sacrifice resilience and other key elastomer properties. Preferably, clear elastomers could be made without hurting performance.
Polyurethane elastomers, in the purest sense, use diol chain extenders and have urethane (--O--CO--NH--) groups but do not have urea (--NH--CO--NH--) groups. In contrast, polyurethane/urea elastomers, because they are made with diamine chain extenders (usually an aromatic diamine), have both urethane and urea groups. So far, polyurethane/urea elastomers have not significantly penetrated the in-line skate market. This is probably because the high viscosities of prepolymers based on PTMEG or polyester polyols make the elastomers hard to process and prevent formulators from easily reaching the targeted hardness.
Recently issued U.S. Pat. No. 5,646,230 summarizes various approaches now used in the field to make polyurethane/urea elastomers and identifies some important concerns. For example, it cites U.S. Pat. No. 3,115,481 to illustrate that isocyanate-terminated prepolymers made from aromatic diisocyanates are usually too reactive with aromatic diamine chain extenders to allow processing of cast elastomers. U.S. Pat. No. 3,997,514 resolves the reactivity problem by making hydroxy-terminated prepolymers from an excess of glycol (e.g., PTMEG) and aromatic diisocyanate. The hydroxy-terminated prepolymer is then reacted with an aliphatic diisocyanate to make an aliphatic diisocyanate-terminated prepolymer that has more controlled, reduced reactivity with aromatic diamines.
In contrast, the '230 patent teaches to make aliphatic diisocyanate-terminated prepolymers by reacting, preferably in a single step, a polyether polyol (PPGs or PTMEG), an aromatic diisocyanate, and an aliphatic diisocyanate. These prepolymers are then chain extended with aromatic diamines. While both the aromatic and aliphatic diisocyanates react into the prepolymer, the aromatic diisocyanate reacts faster so the terminal NCO groups derive primarily from the aliphatic diisocyanate. A relatively large proportion (compared with the amount of aromatic diisocyanate) of the more-expensive aliphatic diisocyanate is used. Missing from the '230 patent is any discussion of how to make clear elastomers while maintaining the high resilience needed for high-quality, in-line skate wheels.
In sum, the high-performance elastomer market would benefit from improved elastomers. Of particular value is a way to make the clear elastomers now in vogue without sacrificing important mechanical properties. Preferably, the elastomers could be made from readily available materials that are listed under the Toxic Substances Control Act (TSCA). Preferably, the process used to make them would avoid the reactivity problems of aromatic diisocyanate-terminated prepolymers and the viscosity issues of prepolymers made from PTMEG and polyester polyols. An ideal process would give clear, low-cost elastomers with an excellent overall balance of properties, particularly high resilience.